Me, I'm hoping rfactor2 gets to a state where I can use it exclusively for MP racing, and any other car games I buy can be for SP/fun.
The way rfactor is supposed to work is that the initial stuff isn't the best, and then x amount of mods filter thru and hey presto, you have a game, however sim racing isn't about 100's of cars, I don't have time to practice with, set up and competitively race 150 cars....impossible, and tbh, totally undesirable.
The reason why GT5/FM4 have so many cars is because they know that no single car is worthy of doing 1000's of laps on as the difficulty level isn't there, nor is the feeling of satisfaction of finding a few 10ths every so often, and in a competitive MP game, a few 10ths is huge.
The F1 redbull is a damn good effort, 250-260kph cornering if you know how to drive, otherwise hello guard rail and a healthy bump on your insurance policy.
Sim racers will enter a online game, and practice for 50-120 laps, and maybe not even bother to race, but they will have improved their times and honed their set up, ready for the opportunity to pounce on GT5 noobs.
Now that I have a decent wheel{G27} I have a hard time seeing the value of arcade games, especially as most of them suck on MP thx to their console origins, and don't offer me the driving thrill/reward{PC sims have the best FFB and steering precision}.
So I have my eye on GTR3, Assetto Corsa, and Reiza's Ayrton Senna game, but only GTR3 stands a chance of having a busy longterm MP, so for me, it'll be down to rfactor or GTR3 for my racing future.